1. Field of the Invention
The present invention relates to thermoelectric materials and devices. More particularly, the present disclosure describes a class of thermoelectric semiconducting and semi-metallic alloys with a filled skutterudite structure and applications thereof for thermoelectric devices.
2. Description of the Related Art
Thermoelectric materials are a class of materials that can efficiently convert between thermal energy and electrical energy. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The Peltier effect is related to the Seebeck effect and is a phenomenon in which heat absorption accompanies the passage of current through the junction of two dissimilar materials. The Peltier effect is used in thermoelectric refrigeration or other cooling applications. In addition, thermoelectric materials are used in heating applications and thermoelectric sensing devices.
Only certain materials have been found usable with these effects, which has limited the ability to use this effect.
Some thermoelectric materials are semiconducting or semi-metallic. These materials conduct electricity by using two types of carriers: electrons and holes. When one atom in a crystal is replaced by another atom with more valence electrons, the extra electrons from the substituting atom are not needed for bonding and can move around throughout the crystal. A semiconductor is called n-type if the conducting carriers are electrons. On the other hand, if an atom in the crystal is replaced with an another different atom having fewer valence electrons, one or more bonds are left vacant and thus positively charged xe2x80x9cholesxe2x80x9d are produced. A semiconductor is called p-type if the conducting carriers are holes.
In the above-mentioned thermoelectric devices, both n-type and p-type thermoelectric materials are usually needed.
Thermoelectric devices can have distinct advantages in many applications. For example, an electric power generator based on thermoelectric materials does not use moving parts like conventional power generators. This feature significantly enhances the reliability of the thermoelectric devices by avoiding mechanical wear of the moving parts and corresponding failure. This further reduces the cost of maintenance. Thermoelectric devices allow operations in hostile environments such as in high temperature conditions (e.g., 900xc2x0 C.) without human attendance. The unique properties of thermoelectric materials also make the thermoelectric devices environmentally friendly, i.e., industrial heat waste or natural heat sources can be used to generate electric power.
The efficiency of a thermoelectric material is often characterized by a thermoelectric figure of merit, ZT. The figure of merit ZT is a dimensionless parameter and is conventionally defined as:                               ZT          =                                                    S                2                            ⁢              σ              ⁢                              xe2x80x83                            ⁢              T                        κ                          ,                            (        1        )            
where S, "sgr", xcexa, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. The larger the ZT, the higher the energy conversion efficiency of a thermoelectric material. An efficient thermoelectric material should have a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity.
Much interest in thermoelectricity was shown between 1957 and 1963 because of the expectations that a high thermoelectric energy conversion efficiency could be achieved and results transferred to large-scale applications. At that time, bismuth telluride (Bi2Te3) and lead telluride (PbTe) were found among the most efficient thermoelectric materials. Many companies and laboratories were involved in the search for better thermoelectric materials. Later on, Si1xe2x88x92xGex alloys were added as a prime material for high-temperature space applications. By optimizing the doping level and the composition of state-of-the-art materials, significant improvements were obtained and maximum ZT values close to 1 were reproducibly achieved.
Numerous thermoelectric materials have been synthesized and their properties were investigated. However, the search for materials which combine high electrical conductivity, high Seebeck coefficient and low thermal conductivity did not result in any breakthroughs.
For the entire temperature range of xe2x88x92100xc2x0 C. to 1000xc2x0 C., maximum ZT of conventional thermoelectric materials are limited to values of about 1, which were supported by the experimental results achieved at that time. Some workers in the art believed that a ZT of 1 may be a limit common to all thermoelectric materials. However, theoretical attempts to determine such a boundary condition for the dimensionless figure of merit ZT have been unsuccessful so far.
In addition to the low conversion efficiency found in the previous thermoelectric materials, the cost to synthesize these materials is high and thus commercial applications of such devices are often not viable. Furthermore, for the state-of-art thermoelectric materials such as PbTe and Bi2Te3 alloys, the number of isostructural compounds is limited and the possibilities to optimize their properties for maximum performance at different temperatures of operation are also limited.
A systematic search for advanced thermoelectric materials was initiated at the Jet Propulsion Laboratory (JPL) several years ago and resulted in the discovery of a new family of promising semiconducting materials with the skutterudite crystal structure.
Skutterudite structure was originally attributed to a mineral from Skutterud of Norway that has a general formula TPn3, in which element T can be Co, Rh, or In and Pn can be P, As or Sb. The unit cell of the skutterudite structure (prototype CoAs3) iscubic space groupIm3 and has a square radical [As4]4xe2x88x92. This anion located in the center of the smaller cube is surrounded by eight Co3+ cations. The unit cell was found to have eight smaller cubes that are often called octants. Two of the octants do not have the anions in the center. This is desirable to maintain the ratio Co:[As4]=4:3 so that the total structure remains electrically neutral and semiconducting. Thus, a typical skutterudite structure results from the Co8[As4]6=2Co4[As4]3 composition and has thirty-two atoms per unit cell.
FIG. 1 shows a typical skutterudite crystal lattice structure. Transition metal atoms 110 form a cubic lattice 112. Non-metal pnicogen atoms 120 form a four-member planary ring 122 which is disposed within the cubic lattice structure 112. Each transition metal atom 110 has six neighboring transition metal atoms 110. Each pnicogen atom 120 has two adjacent pnicogen atoms 120 and two transition metal atoms 110. The covalent bonding associated with a skutterudite-type crystal lattice structure provides high carrier mobility. The complex structure and heavy atoms associated with skutterudite-type crystals also result in relatively low thermal conductivity. These two properties in combination are desirable in improving thermoelectric properties in new semiconductor materials.
Various skutterudite structure materials have been investigated for applications in thermoelectric devices. It is known in the art that high carrier mobility values are usually found in crystal structures with a high degree of covalency. The bonding in a skutterudite structure has been found to be predominantly covalent. Moreover, high hole mobility values have been measured in several skutterudite compounds including IrSb3, RhSb3, CoSb3, and RhP3.
In addition, thermoelectric materials with a filled skutterudite crystal structure have also been synthesized. The chemical composition of these types of compounds can be represented by the following formula for half of the unit cell:
LnT4Pn12xe2x80x83xe2x80x83(2)
where Ln includes rare earth elements such as La, Ce, Pr, Nd, Sm, Eu, Gd, Th, and U; T includes transition metal elements such as Fe, Ru, and Os; and Pn includes non-metal atoms such as pnicogen elements P, As, and Sb. The empty octants of the skutterudite, which are formed in the TPn3(xcx9cT4Pn12) framework, are filled with a rare earth element. Because the T4Pn12 groups using Fe, Ru or Os are electron-deficient relative (by 4exe2x88x92) to the unfilled skutterudite electronic structure that uses Co, Rh, or Ir, the introduction of the rare earth atoms compensates this deficiency by adding free electrons. However, the number of valence electrons contributed by the rare earth atoms is generally insufficient. For example, La has 3+ oxidation states, and Ce can be 3+ or 4+. Therefore, most of these filled skutterudite compounds behave as metals, or very heavily doped p-type semi-metals.
Skutterudites seem promising for highly efficient thermoelectric materials. This is in part due to their large mobility values. A typical unit cell in these compounds is relatively large with 32 to 34 atoms and has a cubic geometry. The electric properties of binary skutterudite materials are attractive for thermoelectric applications. However, thermal conductivity of these binary skutterudites at room temperature is in an approximate range from 100 mW cmxe2x88x921 Kxe2x88x921 to 150 mW cmxe2x88x921 Kxe2x88x921. This is too high and makes high ZT values difficult to achieve since ZT is inversely proportional to the thermal conductivity as shown by Equation (1). Substantial reductions in the lattice thermal conductivity are desirable to achieve ZT values comparable to those of state-of-the-art thermoelectric materials which is in a range of 10 mW cmxe2x88x921 Kxe2x88x921xcx9c40 mW cmxe2x88x921 Kxe2x88x921.
The inventors have devised different approaches to solve the problem.
One approach is to use binary compounds with high carrier mobility to form solid solutions with other binary compounds or a new ternary/quaternary isostructural phase using the transition metal and/or the pnicogen site(s).
Another approach is to prepare new ternary and quaternary skutterudite phases, derived from the binary compounds by substituting the transition metal element and/or the pnicogen element with elements from adjacent columns of the periodic table. Both these approaches have been disclosed in U.S. Pat. Nos. 5,610,366 and 5,747,728.
The present application further discloses another new class of advanced thermoelectric materials, filled skutterudite compositions with a variety of atomic substitutions and combinations. This is a continuation of the above referenced U.S. patent applications in developing new high-efficiency thermoelectric materials and devices.
According to the present application, the thermal conductivity of a skutterudite can be reduced by filling the two empty octants present in the 32-atom unit cell of a binary compound and in addition substituting elements to replace part of the original transition metal and/or pnicogen elements to conserve the valence electron count of the unit cell. This novel filled skutterudite structure is believed to lead to a new class of thermoelectric materials of high ZT values in a wide temperature range and many engineering versatilities.
The inventors recognized that the unique structure of skutterudite crystals has the potential to achieve high electrical conductivity and low thermal conductivity for highly efficient thermoelectric materials.
In particular, the inventors recognized that a heavy filling atom in a filled skutterudite structure can effectively scatter phonons so as to substantially reduce the lattice thermal conductivity of the unfilled compound. The inventors recognized, importantly, that the filling atom in an empty octant will not substantially decrease the high carrier mobility, which is desirable in achieving high ZT values in such compounds.
The inventors recognized the importance of maintaining the semiconducting properties of a filled skutterudite crystal to achieve high Seebeck coefficients. The inventors recognized that substituting atoms can be used to modify the carrier concentration and to further increase the phonon scattering to reduce the thermal conductivity.
The inventors also recognized that both doping level and conductivity type (i.e., n-type or p-type ) in a filled skutterudite structure can be controlled by changing the ratio between the substituting atoms and the filling atoms.
One aspect of the present invention is a novel structure of filled skutterudite compounds with substituting atoms. This new class of compounds is thermoelectric materials with high ZT values, low thermal conductivity and high electrical conductivity. Examples of such new materials which have been prepared in accordance with the present invention include, but are not limited to, CeFe4Sb12, CeRu4Sb12, CeFe4As12, CeRu4As12, CeFe4xe2x88x92xCoxSb12, CeFe4xe2x88x92xNixSb12, CeFe4xe2x88x92xRuxSb12, CeFe4Sb12xe2x88x92yAsy, LaFe4Sb12, and CeFe4GeSb11, in which 0xe2x89xa6xxe2x89xa64 and 0xe2x89xa6yxe2x89xa612.
Another aspect of the present invention is the use of many substituting techniques to construct a variety of filled skutterudites with different desired properties. For example, one such technique is replacing a pnicogen element or a transition metal element in a filled skutterudite with a different main-group element such as an element from columns 14, 15, 16 of the periodic table or a different transition metal element in the same row of the periodic table and adjusting the carrier concentration thereof. Examples of such compounds include CeFe4xe2x88x92xNixSb12 and CeFe4GexSb12xe2x88x92x for 0xe2x89xa6xxe2x89xa612. Another substituting technique uses an xe2x80x9calloyingxe2x80x9d technique to substitute a pnicogen element or a transition metal element in a filled skutterudite with a different pnicogen or transition metal element in the same column of the periodic table, e.g., CeFe4xe2x88x92xRuxSb12 for 0xe2x89xa6xxe2x89xa64 and CeFe4Sb12xe2x88x92yAsy for 0xe2x89xa6yxe2x89xa612. An element in the structure may also be replaced by a different element at a different row and different column in the periodic table. Furthermore, the concentration of a filling element can be varied, or more than one filling element can be used to achieve desired filled skutterudites. Examples of this type of compound include CeyFe4xe2x88x92xNixSb12 for 0xe2x89xa6xxe2x89xa64 and 0xe2x89xa6yxe2x89xa61 or Ce1xe2x88x92xEuxFe4Sb12 for 0xe2x89xa6xxe2x89xa61.
Another aspect of the invention is the preparation of the such new semiconductor compounds by using an economic and efficient method to facilitate the commercialization of the invention. In particular, the present invention discloses a synthesizing process to form polycrystalline filled skutterudite compositions for thermoelectric devices.
Yet another aspect of the invention is using such new materials in a variety of thermoelectric devices for electric power generation, heating applications, cooling applications, and sensing devices. For example, the weight, volume, cost of thermoelectric power generators for spacecraft used in deep space missions need to be reduced, and thermoelectric materials which can achieve thermoelectric conversion efficiency better than about 13% are desirable. In terrestrial applications, such new thermoelectric power generators preferably can work with a heat source of 600xc2x0 C. to 800xc2x0 C. as in heat recovery from a processing plant of combustible solid waste. Generating electric power from waste exhaust heat (about 400xc2x0 C. to 700xc2x0 C.) to supplement or replace the alternator in automobiles is another potential application in reducing fuel consumption.